ReleasePageStream

public delegate void ReleasePageStream(int pageNumber, Stream pageStream);

    CreatePageStream createDelegate = delegate (int pageNumber)
    {
        var pagePath = System.IO.Path.Combine(@"C:\Temp", string.Format("page_{0}.png", pageNumber));
        return System.IO.File.Create(pagePath);
    };
    ReleasePageStream releaseDelegate = delegate (int pageNumber, System.IO.Stream pageStream)
    {
        // do anything with Stream, containing page preview
        pageStream.Close();
    };
    var previewOptions = new PreviewOptions(createDelegate, releaseDelegate);
    previewOptions.PreviewFormat = PreviewOptions.PreviewFormats.PNG;
    previewOptions.Height = 640;
    previewOptions.Width = 480;
    previewOptions.PageNumbers = new int[] { 1 };
    using (var redactor = new Redactor("C:\Temp\SourceFile.pdf"))
    {
        redactor.GeneratePreview(previewOptions);
    }